Application Ser. No. 150,054, filed June 4, 1971 now U.S. Pat. No. 3,707,309, discloses a hydraulic brake system for railway rapid transit service which is known commercially as the PS-68 Brake Equipment marketed by the New York Air Brake Company, a Unit of General Signal Corporation. In this system, the brake cylinders on each truck are provided with a translating and valving unit of the type described in U.S. Pat. No. 3,622,207, granted Nov. 23, 1971, which joins the cylinders to a hand brake control unit and to an air-to-hydraulic booster constructed in accordance with the preferred teachings of U.S. Pat. No. 3,513,656, granted May 26, 1970. Although this system is practical and affords many advantages, it does have the following undesirable characteristics:
1. The booster has a two-stage discharge stroke which enables it to effect take-up of shoe clearance with minimum consumption of air. This feature is desirable, particularly in the case of brakes used on freight cars and locomotives where conservation of compressed air can be a necessity. However, since the time required for the booster to shift from the low pressure to the high pressure discharge stage does delay somewhat applications of the brakes, the desirable feature is accompanied by an adverse side effect. In the case of rapid transit service, this side effect can be intolerable, and, in any event, probably outweighs reduction in air consumption.
2. The booster affords double-acting slack compensation to the service brake, whereas the translating and valving unit affords only slack take-up compensation to the hand brake. Therefore, the increased shoe clearance needed for brake shoe or wheel replacment is created by cycling of the service brake. This procedure, of course, requires the availability on the car of compressed air. Since the reservoirs on the car are not always charged during this type of maintenance, the procedure sometimes can be burdensome.
3. If shoe replacement is not followed immediately by cycling of the service brake, shoe clearance can be reduced to the point where thermal expansion of the hydraulic fluid will develop substantial pressures in the brake cylinders. The system includes no provision for relieving such pressures, and therefore the car may be reintroduced into service with the shoes in tight contact with the wheels. This stuck brake condition obviously is undesirable.
4. The system employs relatively complex components which are heavy and require many parts.
One object of the present invention is to provide an improved hand brake-service brake system which eliminates the undesirable characteristics mentioned above. According to the invention, the new system includes a simple single stage booster, and a separate hydraulic slack-adjusting unit which is selectively connected with the booster or the hand brake control unit by a double check valve and which, therefore, serves both the service brake and the hand brake. This arrangement eliminates the time delay inherent in the two-stage booster and effects a substantial reduction in the number of parts and in the weight of the equipment. It also permits a significant reduction in the size of the reservoir connection of the booster. The slack adjuster unit allows take-up of excess shoe clearance during cycling of either brake, and, under release conditions, it permits transfer of fluid from the brake cylinders to the system reservoir whenever cylinder pressure exceeds a predetermined, relatively low level. This relief feature not only precludes the stuck brake condition mentioned above, but also permits creation of the increased shoe clearance needed for shoe or wheel replacement by the simple expedient of manually retracting the brake cylinder with a pry bar. In its preferred form, the slack adjuster affords true double-acting slack compensation, so it corrects for insufficient as well as excess shoe clearance during cycling of either brake. This feature enables the system to maintain a predetermined shoe clearance throughout the service life of the shoes. Moreover, it permits shoe clearance to be increased intentionally during maintenance operations by cycling of either of the brakes.
The booster-slack adjuster combination is useful by itself to control the brake cylinders on trucks which do not require hand braking. Preferably, these two components are packaged in a unit which is adapted to accept, as alternative constituents, a cartridge version of the double check valve or a sealed plug. In this way, the same basic unit can be adapted for use in either a combined service brake-hand brake system or an exclusive service brake system by simply substituting the plug or the cartridge for the other alternative.
The slack adjuster unit provided by the invention can be used with any source of hydraulic pressure, and thus has utility outside the field of railway brakes. For example, the unit can be built in small sizes and used between the master cylinder and the wheel cylinders of an automobile to maintain brake adjustment. This application is attractive because it eliminates the need for the complex mechanical devices now employed for this purpose. The unit can also be used to maintain very close clearances in either railway or automotive brakes of the disc type. The last mentioned application has special appeal because, with clearances on the order of a few thousandths of an inch, the energy requirements of the brake can be satisfied by a master cylinder small enough to obviate the vacuum booster now being used in automotive disc brake systems.